Dielectric Film Coating for Full Conversion Ceramic Platelets

ABSTRACT

Furthermore, the present invention is directed to a light emitting device assembly and methods for preparing a wavelength converter and methods for preparing a light emitting device assembly.

TECHNICAL FIELD

This invention relates to wavelength converters, light emitting deviceassemblies and methods for preparing wavelength converters and lightemitting devices.

BACKGROUND

There are several embodiments of phosphor wavelength converters in theprior art. Often ceramic phosphor materials are used to convert light ofa certain wavelength to a certain further wavelength. However, some ofthe ceramic phosphor materials show a scattering.

For example, amber ceramics with the composition (Sr,Ba)₂Si₅N₈:Eu areused for full conversion LED applications, where the blue light from ablue LED die is completely or nearly completely absorbed by an amberceramic platelet converter on top of the LED die, and re-emitted atlonger wavelengths as amber light. The full conversion amber LEDs havebroad applications, such as automotive turn signal and tail lights,emergency vehicle signal lights and traffic signal lights.

In the past, the efforts to improve amber ceramic efficacy have beenfocused on improving densification of amber ceramics. By reducingporosity, the portion of scattering from pores is reduced. However, asthe crystal structure of (Sr,Ba)₂Si₅N₈:Eu is not cubic, there is alwaysgrain boundary scattering caused by birefringence. Also it is difficultto get rid of a Ba₁Si₇N₁₀ secondary phase completely, as the scatteringfrom the secondary phases always exists. So there is a limitation inimproving efficacy by reducing porosity because scattering from grainboundary and second phases cannot be avoided.

U.S. Pat. No. 8,957,493 discloses a light emitting diode (LED) assemblycomprising a layer of a wavelength converter and a filter layer.

Oh et al., Optics Express 2010, 18(11), 11063-11072 discloses anamber-phosphor LED with comprising a filter layer.

SUMMARY

It is an object of the present invention to obviate the disadvantages ofthe prior art.

It is another object of the present invention to provide a wavelengthconverter that might be used in LED applications.

It is a further object of the present invention to provide a method forpreparing a wavelength converter and for providing a light emittingdevice.

It is also an object of the present invention to provide a wavelengthconverter and a light emitting device prepared by a method according tothe present invention.

In accordance with one object of the present invention, there isprovided a wavelength converter comprising:

a phosphor layer and

a filter layer,

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of betweenabout 20 μm to about 80 μm.

In accordance with another object of the present invention, there isprovided a light emitting device assembly comprising:

a LED die, and

a wavelength converter comprising:

-   -   a phosphor layer and    -   a filter layer,

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of betweenabout 20 μm to about 80 μm.

In accordance with another object of the present invention, there isprovided a method for preparing a wavelength converter comprising thesteps:

providing a glass substrate or a sapphire wafer,

coating the glass substrate or the sapphire wafer with a filter layercomprising different metal oxides to prepare a coated glass substrate orcoated sapphire wafer,

providing a phosphor material, and

attaching the coated glass substrate or coated sapphire wafer to thephosphor material, thereby providing a wavelength converter

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of betweenabout 20 μm to about 80 μm.

In accordance with another object of the present invention, there isprovided a method for preparing a light emitting device assembly,comprising:

providing a LED die, optionally attached to a leadframe,

attaching a wavelength converter comprising:

-   -   a phosphor layer and    -   a filter layer    -   to the LED die,

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of betweenabout 20 μm to about 80 μm.

In accordance with another object of the present invention, there isprovided a wavelength converter prepared by a method according to thepresent invention.

In accordance with another object of the present invention, there isprovided a light emitting device prepared by a method according to thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below on the basis of theexamples and with reference to the associated figures. The figures arediagrammatic and do not represent illustrations that are true to scale.

FIG. 1 shows a SEM picture of (Sr,Ba)₂Si₅N₈:Eu amber ceramic;

FIG. 2 shows lumens of (Sr,Ba)₂Si₅N₈:Eu amber ceramic LEDs vs theceramic platelet thickness;

FIG. 3 shows the color of (Sr,Ba)₂Si₅N₈:Eu amber ceramic LEDs vs theceramic thickness;

FIG. 4 shows the color of (Sr,Ba)₂Si₅N₈:Eu amber ceramic platelets withvarious thicknesses measured with OSRAM tester;

FIG. 5 shows reflectance of a filter layer according to Table 1 andemission of an amber platelet before and after the coating;

FIG. 6 shows the color of (Sr,Ba)₂Si₅N₈:Eu amber ceramic platelets withvarious thicknesses before and after coating measured with OSRAM tester;

FIG. 7 shows the color of (Sr,Ba)₂Si₅N₈:Eu coated amber ceramicplatelets in a LED package;

FIG. 8 shows the lumens of coated amber ceramic platelets in a LEDpackage;

FIG. 9 shows a SEM image of a cross section of a coating on an amberceramic plate;

FIG. 10 shows the color of coated amber ceramic platelets withunpolished and polished surfaces tested with OSRAM tester measurement;

FIG. 11 shows a schematic view of a filter layer on a transparent layeron top of an amber ceramic plate;

FIG. 12 shows a schematic view of a filter layer on a transparent layeron top of an amber ceramic in a LED package;

FIG. 13 shows an exemplary method for preparing a wavelength converter;

FIG. 14 shows an exemplary method for preparing a light emitting deviceassembly;

FIG. 15 shows a schematic view of light and heat transmission in a thickceramic;

FIG. 16 shows a schematic view of light and heat transmission in a thinceramic;

FIG. 17 shows the spectral reflective properties of the filter layer atdifferent angles of incidence;

FIG. 18 shows the color of (Sr,Ba)₂Si₅N₈:Eu amber ceramic LEDs as seenfrom different observation angles for (1) a thin ceramics layer withfilter layer and (2) thick ceramics layer without filter; and

FIG. 19 shows exemplary embodiments of wavelength converters.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

References to the color of the phosphor, LED, or conversion materialrefer generally to its emission color unless otherwise specified. Thus,a blue LED emits a blue light, a yellow phosphor emits a yellow lightand so on.

The present invention is directed to a wavelength converter comprising:

a phosphor layer and

a filter layer,

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of between 20μm to 80 μm.

As used herein a wavelength converter is a solid structure that convertsat least part of the light of a certain first wavelength to light of acertain second wavelength. In an embodiment, light of a certain firstwavelength is blue light. Structures that can produce light of a firstwavelength are, e.g., InGaN or GaN chips, or solid state laser diodes.

A phosphor is a material that converts light of a certain firstwavelength to light of a certain second wavelength.

According to the present invention, the wavelength converter comprises aphosphor layer. The phosphor layer may consist of the phosphor material,or alternatively, comprise a host material in which the phosphor isintercalated. In the latter case, the phosphor might be intercalated ina ceramic host material or a glass host material. Preferably, the hostmaterials are transparent for the incoming light of a certain firstwavelength. The phosphor layer might also be a phosphor ceramic layer.

The phosphor layer (e.g., the phosphor ceramic layer) might have asmooth surface, preferably a polished surface.

Exemplary phosphors are garnets, oxynitridosilicates, perovskits,quantum dots, silicates or combinations thereof, each doped with atleast one appropriate element.

In an embodiment, the phosphor is selected from the group consisting of(Ba,Sr)₂Si₅N₈:Eu²⁺, Ca-α-SiAlON:Eu²⁺, YAG:Ce+CaAlSiN₃:Eu,(Ca,Sr)AlSiN₃:Eu²⁺, (Sr,Ca)Al₂Si₂N₆:Eu²⁺, (Ba,Sr,Ca)₂Si₅N₈:Eu²⁺ andSr(LiAl₃N₄):Eu²⁺.

The phosphor (e.g., the phosphor mentioned herein) can be mixed with aceramic host material selected from the group consisting of undoped(Ba,Sr)₂Si₅N₈:Eu²⁺, Ca-α-SiAlON or AlN, or a glass host materialconsisting of low melting temperature glass, borate silicate orphosphate glass.

In preferred embodiments, the phosphor layer is a layer of(Ba,Sr,Ca)₂Si₅N₈:Eu²⁺, or (Ba,Sr)₂Si₅N₈:Eu²⁺, i.e., a layer of amberceramic. In a further preferred embodiment, the phosphor layer is(Ba,Sr)₂Si₅N₈:Eu²⁺.

In an alternative embodiment, the phosphor layer comprises at least twodifferent phosphors, at least three different phosphors, or at leastfour different phosphors.

In an embodiment, the certain second wavelength, i.e., the dominantwavelength that is obtained from the conversion of the certain firstwavelength is about 590 nm. The phosphors that might convert the lightto this wavelength are, e.g., (Ba,Sr)₂Si₅N₈:Eu²⁺, Ca-α-SiAlON:Eu²⁺ andYAG:Ce+CaAlSiN₃:Eu. The phosphors might be present as a ceramic layer,or might be present as a particle in a host material, such as atransparent ceramic, glass or silicone.

In an alternative embodiment, the certain second wavelength, i.e., thewavelength that is obtained from the conversion of the certain firstwavelength is about 610 nm to about 630 nm. The phosphors that mightconvert the light to this wavelength are, e.g., (Ca,Sr)AlSiN₃:Eu²⁺,(Sr,Ca)Al₂Si₂N₆:Eu²⁺, (Ba,Sr,Ca)₂Si₅N₈:Eu²⁺, Sr(LiAl₃N₄):Eu²⁺. Thephosphors might be present as a ceramic layer, or might be present as aparticle in a host material, such as a transparent ceramic, glass orsilicone.

In an alternative embodiment, the certain second wavelength, i.e., thewavelength that is obtained from the conversion of the certain firstwavelength is more than about 700 nm. The phosphors that might convertthe light to this wavelength are, e.g., Cr^(3+/4+), Ni²⁺, Bi, Yb, Tm, Eretc. doped host crystals and glasses (e.g., La₃Ga₅GeO₁₄, Ga₂O₃,Gd₃Ga₃Sc₂O₁₂, Mg₂SiO₄, etc.). The phosphors might be present as aparticle in a host material, such as a transparent ceramic, glass orsilicone.

The phosphor layer has preferably a shape-like structure. The thicknessof the phosphor layer, i.e., the length of the phosphor layer throughwhich the light traverses in a 90° angle with respect to the emittingsource surface of the light of the certain first wavelength, is, e.g.,between 420 to 465 nm, preferably 445 to 455 nm.

In an embodiment, there is more than one phosphor layer, e.g., there are2 layers, 3 layers or more layers.

The wavelength converter further comprises a filter layer on top. Thefilter layer reflects unabsorbed excitation light of a certain firstwavelength which is emitted, e.g., from a LED die. The phosphor layerabsorbs at least part of the light of the certain first wavelength andconverts it to a certain second wavelength and the filter layer reflectsthe unabsorbed light of a certain first wavelength, whereas the light ofthe certain second wavelength is transmitted.

The filter layer is directly attached to the phosphor layer. “Directlyattached” in the context of the present invention means that there is atleast one contact point between the phosphor layer and the filter layer.In a preferred embodiment at least one surface of the phosphor layer iscovered with the filter layer.

The filter layer of the present invention may be a multi-layer ofalternative oxides. The filter may be made by vapor or sputteringdeposition of these oxide layers on a substrate. The substrate can be aphosphor layer, e.g., in ceramic form. In this case, there is preferablyno glue in between the phosphor layer and the filter layer.Alternatively the substrate can be a thin glass or sapphire layer. Inthis case, the coated glass (or sapphire) is the filter layer. Thefilter layer is preferably glued to the phosphor layer.

In an embodiment, the filter layer comprises at least two metal oxides.The metal oxides of the filter layer might be present in one layer. Inan alternative aspect of this embodiment, the filter layer comprisesseveral sub-layers. Each of the sub-layers might comprise at least onemetal oxide.

The filter layer might be a dichroic filter. A dichroic filter,thin-film filter, or interference filter is a very accurate color filterused to selectively pass light of a small range of colors whilereflecting other colors.

In a dichroic mirror or filter alternating layers of optical coatingswith different refractive indices are built up upon a glass substrate.The interfaces between the layers of different refractive index producephased reflections, selectively reinforcing certain wavelengths of lightand interfering with other wavelengths. The layers are usually added byvacuum deposition. By controlling the thickness and number of thelayers, wavelength of the passband of the filter can be tuned and madeas wide or narrow as desired. Because unwanted wavelengths are reflectedrather than absorbed, dichroic filters do not absorb this unwantedenergy during operation and so do not become nearly as hot as theequivalent conventional filter (which attempts to absorb all energyexcept for that in the passband).

The filter layer might comprise, e.g., two sub-layers, each comprising adifferent metal oxide. Preferably the metal oxides have contrastrefractive indices. In an alternative embodiment, the filter layercomprises three or more sub-layers, each comprising a different or thesame metal oxide, wherein preferably sub-layers of the same metal oxideare not attached to each other. Therefore, the filter layer mightcomprise n sub-layers and up to n different metal oxides. In a preferredembodiment, the filter comprises two metal oxides, one with a highrefractive index and one with a low refractive index.

In an embodiment, the filter layer comprises 13 to 19 sub-layers. In apreferred embodiment, the filter layer comprises 15 to 18 sub-layers. Inan even more preferred embodiment, the filter layer comprises 16 or 17sub-layers. In a further preferred embodiment, the filter layercomprises 17 sub-layers.

The metal oxides of the filter layer might be selected from SiO₂, Al₂O₃,TiO₂, Nb₂O₅, Ta₂O₅, HfO₂ and Y₂O₃. Preferred metal oxides are pairs ofAl₂O₃—TiO₂ or SiO₂— Nb₂O₅.

Dichroic filters normally need two oxides with a contrast refractiveindex, one has high refractive index, H, and the other has lowrefractive index, L. The filter preferably comprises alternative H, Llayers with different thicknesses, such as H, L, H, L, H, L, H, L, H, L,etc.

In an embodiment, the filter layer comprises 17 sub-layers and twodifferent metal oxides, such as alternating sub-layers of Al₂O₃ andTiO₂.

The filter layer has preferably a shape-like structure. The thickness ofthe filter layer, i.e., the length of the filter layer through which thelight traverses in a 90° angle with respect to the emitting sourcesurface, of the light of the certain first wavelength, is, e.g., between420 to 465 nm, preferably 445 to 455 nm.

The sub-layers of the filter layer might each have a thickness ofbetween about 20 nm to about 150 nm. In an embodiment, the sub-layerseach have a thickness of between about 30 nm to about 40 nm.

The filter layer thickness is preferably related to the wavelength λ.The thickness is multiple times of ½ λ. In the present invention, a highreflectance at a wavelength below cutoff wavelength, such as below 535nm, in the blue excitation light; and high transmission above cutoffwavelength, in the amber phosphor emission region should be reached.

The wavelength converter might further comprise an absorption layerabove the filter. The absorption layer preferably absorbs light ofwavelengths which shall not pass through the filter layer to be emitted.

Examples of absorption layers are layers of ion doped color filterglasses, comprising MoS₂ dyed glasses, Ce-doped Gallium-Gadolinium-YAG,or are selected from semiconductor materials like GaP, AlP, AlAs, CdSe,CdS in the form of thin layers or nanoparticles.

The absorption layer is preferably attached to the filter layer. Theabsorption layer might be attached to the filter layer with a glue ormight be attached due to inherent absorption forces.

The filter layer has preferably a shape-like structure. The thickness ofthe filter layer, i.e., the length of the filter layer through which thelight traverses in a 90° angle with respect to the emitting sourcesurface of the light of the certain first wavelength, is, e.g., betweenabout 420 nm to about 465 nm, preferably about 445 nm to 455 nm.

The wavelength converter has an overall thickness of between about 20 μmto about 80 μm. In an embodiment, the wavelength converter has athickness of between about 40 μm to about 70 μm. In a preferredembodiment, the wavelength converter has a thickness of between about 40μm to about 50 μm. The thickness of the wavelength converter is thelength of the wavelength converter through which the light traverses ina 90° angle with respect to the emitting source surface of the light ofthe certain first wavelength.

It is a further object of the present invention to provide a lightemitting device assembly comprising:

a LED die, and

a wavelength converter comprising:

a phosphor layer and

a filter layer,

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of betweenabout 20 μm to about 80 μm.

The LED die preferably emits blue light. In an alternative embodiment,the LED die emits UV light. Examples of LED dies are GaN/InGaN basedsemiconductor materials.

The wavelength converter, the phosphor layer and the filter layercorrespond to the respective means as described above.

It is an object of the present invention to provide a method forpreparing a wavelength converter comprising the steps:

providing a glass substrate or a sapphire wafer,

coating the glass substrate or the sapphire wafer with a filter layercomprising different metal oxides to prepare a coated glass substrate orcoated sapphire wafer,

providing a phosphor material, and

attaching the coated glass substrate or coated sapphire wafer to thephosphor material, thereby providing a wavelength converter,

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of betweenabout 20 μM to about 80 μm.

In a step of the method for preparing a wavelength converter a glasssubstrate or a sapphire wafer is provided. The glass substrate or thesapphire wafer is preferably transparent and therefore especiallytransmissive for wavelengths which shall be emitted from the wavelengthconverter.

The glass substrate or the sapphire wafer might have a thickness ofbetween about 40 to about 200 um. In an embodiment, the glass substrateor the sapphire wafer might have a thickness of between about 40 toabout 100 um.

In a further step, the glass substrate or the sapphire wafer is coatedwith at least two layers of metal oxides to provide a coated glasssubstrate or a coated sapphire wafer. The coating is preferably carriedout by depositing a pair of Al₂O₃—TiO₂ or SiO₂— Nb₂O₅ oxides, howeveralso other metal oxides can be deposited on the glass substrate or thesapphire wafer.

In an embodiment, the coating is carried out stepwise. So the firstlayer of at least one metal oxide is coated on the glass substrate orthe sapphire wafer. In a subsequent step, a second layer of at least onemetal oxide is coated on the glass first layer of at least one metaloxide. Further layers, if present, are introduced accordingly. As anoptional step a drying step is present between the coating steps, or atleast after the final coating step. The coating might be carried out byvapor or sputtering deposition.

In an embodiment of the present invention the layers of the metal oxidescorrespond to the sub-layers of the filter layer as mentioned herein andthe sum of all layers of metal oxides that are coated on the glasssubstrate or the sapphire wafer correspond to the filter layer asmentioned herein.

In an embodiment, the metal oxides are selected from SiO₂, Al₂O₃, TiO₂,Nb₂O₅, Ta₂O₅, HfO₂ and Y₂O₃. Preferred metal oxides are pairs ofAl₂O₃—TiO₂ or SiO₂— Nb₂O₅ oxides.

In an embodiment, there are 17 layers of metal oxides coated on theglass substrate or the sapphire wafer and the metal oxides are Al₂O₃ andTiO₂.

In a further step, a phosphor material is provided. Exemplary phosphorsare garnets, oxynitridosilicates, perovskites, quantum dots, silicatesor combinations thereof, each doped with at least one appropriateelement.

In an embodiment, the phosphor material is selected from the groupconsisting of (Ba,Sr)₂Si₅N₈:Eu²⁺, Ca-α-SiAlON:Eu²⁺, YAG:Ce+CaAlSiN₃:Eu,(Ca,Sr)AlSiN₃:Eu²⁺, (Sr,Ca)Al₂Si₂N₆:Eu²⁺, (Ba,Sr,Ca)₂Si₅N₈:Eu²⁺ andSr(LiAl₃N₄):Eu²⁺.

The phosphor (e.g., the phosphor mentioned herein) can be mixed with aceramic host material selected from the group consisting of undoped(Ba,Sr)₂Si₅N₈:Eu²⁺, Ca-α-SiAlON or AlN, or a glass host materialconsisting of low melting temperature glass, borate silicate orphosphate glass.

In preferred embodiments, the phosphor material is a layer of(Ba,Sr)₂Si₅N₈:Eu²⁺, or (Ba,Sr)₂Si₅N₈:Eu²⁺, i.e., a layer of amberceramic. In a further preferred embodiment, the phosphor material is(Ba,Sr)₂Si₅N₈:Eu²⁺.

In an alternative embodiment, the phosphor material comprises at leasttwo different phosphors, at least three different phosphors, or at leastfour different phosphors.

In an embodiment, the certain second wavelength, i.e., the dominantwavelength that is obtained from the conversion of the certain firstwavelength is about 590 nm. The phosphors that might convert the lightto this wavelength are, e.g., (Ba,Sr)₂Si₅N₈:Eu²⁺, Ca-α-SiAlON:Eu²⁺ andYAG:Ce+CaAlSiN₃:Eu. The phosphors might be present as a ceramic layer,or might be present as a particle in a host material, such astransparent ceramic, glass or silicone.

In an alternative embodiment, the certain second wavelength, i.e., thewavelength that is obtained from the conversion of the certain firstwavelength is about 610 nm to about 630 nm. The phosphors that mightconvert the light to this wavelength are, e.g., (Ca,Sr)AlSiN₃:Eu²⁺,(Sr,Ca)Al₂Si₂N₆:Eu²⁺, (Ba,Sr,Ca)₂Si₅N₈:Eu²⁺, Sr(LiAl₃N₄):Eu²⁺. Thephosphors might be present as a ceramic layer, or might be present as aparticle in a host material, such as a transparent ceramic, glass orsilicone.

In an alternative embodiment, the certain second wavelength, i.e., thewavelength that is obtained from the conversion of the certain firstwavelength is more than about 700 nm. The phosphors that might convertthe light to this wavelength are, e.g., Cr^(3+/4+), Ni²⁺, Bi, Yb, Tm, Eretc. doped host crystals and glasses (e.g., La₃Ga₅GeO₁₄, Ga₂O₃,Gd₃Ga₃Sc₂O₁₂, Mg₂SiO₄, etc.). The phosphors might be present as aparticle in a host material, such as a transparent ceramic, glass orsilicone.

The coated glass substrate or the coated sapphire wafer is attached tothe phosphor material to provide a wavelength converter. In anembodiment, the attaching of the coated glass substrate or coatedsapphire wafer to the phosphor material is carried out by laminating thecoated glass substrate or coated sapphire wafer to the phosphor materialwith a glue.

The glue might be transparent epoxy, silicone, or polysiloxane.

In a preferred embodiment, the phosphor material is attached to themetal oxide layer(s) and the glass substrate or the sapphire wafer isopposite to the phosphor material. In an alternative embodiment, thephosphor layer is attached to the glass substrate, or the sapphire waferand the filter layer is opposite to the phosphor layer.

In a further embodiment, an adsorption layer as mentioned herein isattached to the filter layer.

In an embodiment, the wavelength converter is diced in smaller pieces.Typical sizes of a wavelength converter are 0.75 mm×0.75 mm, 1 mm×1 mmor 2 mm². In a preferred embodiment, the wavelength converter has a sizeof 1 mm×1 mm.

A further object of the present invention is to provide a method forpreparing a light emitting device assembly, comprising:

providing a LED die, optionally attached to a leadframe,

attaching a wavelength converter comprising:

a phosphor layer and

a filter layer

to the LED die,

wherein the filter layer is directly attached to the phosphor layer andwherein the wavelength converter has an overall thickness of betweenabout 20 μm to about 80 μm.

The LED die, the phosphor layer and the filter layer correspond to therespective means as described herein.

In an embodiment, the attaching of the wavelength converter is carriedout by using silicone glue to the LED die.

A further object of the present invention is to provide a wavelengthconverter prepared by a method of the present invention.

A further object of the present invention is to provide a light emittingdevice prepared by a method of the present invention.

In an embodiment of the present invention, a wavelength converter of thepresent invention is used in LED packages for industrial, automotivelighting.

One typical microstructure of (Sr,Ba)₂Si₅N₈:Eu amber ceramic is shown inFIG. 1. The light color matrix phase is (Sr,Ba)₂Si₅N₈. The darker phasesare Ba₁Si₇N₁₀. Some pores also exist in the ceramics. There can be quitea lot of scattering within the amber ceramics. The first scatteringsource is from grain boundary scattering since (Sr,Ba)₂Si₅N₈ is not acubic structure. The secondary scattering is from the Ba₁Si₇N₁₀secondary phases. The third scattering comes from the pores due to alarge refractive index contrast between the pore and the matrix.

Due to the significant scattering often existing in the amber ceramics,the total scattering is dependent on the amber ceramic thickness. Thethicker the samples, the more scattering exists. FIG. 2 shows lumens ata 350 mA drive current of LEDs using amber ceramic platelet converterswith different thicknesses. Ld=447 nm means that the LED die emits bluelight at a dominant wavelength of 447 nm. W/O cast means without whitesilicone casting around the LED die and converter. It shows clearly thatthe LED lumens decrease as the amber ceramic thickness increases. Thisis due to a more scattering reduced package efficacy.

It seems naturally that a thinner amber ceramic would be preferred dueto its higher package lumens. But according to the color of amberceramic LEDs with various thicknesses shown in FIG. 3, the color of thethinnest amber ceramic LEDs falls outside of required colorspecification. The larger dashed line box is the ECE (Economiccommission for Europe) amber color requirement. Although thinner amberceramics give higher package lumens, they cannot absorb all the bluelight and thus have more residual blue light. More blue residual lightleads to the reduction of color Cx. The emission color shifts to higheremission dominant wavelength gradually when the thickness increases.This is due to the self-absorption of thick samples.

The current standard thickness of amber ceramics in wavelengthconverters is about 120 μm. Such a thickness would guarantee the ceramicis thick enough to absorb most of the light of a certain firstwavelength, such as blue light. The platelets thickness was reduced downto about 90 and about 70 μm. The color and conversion efficacy CE weremeasured by an in-house pin hole setup: OSRAM Tester. A stabilized andconstant blue light passes through a pin hole where the platelet samplesits on. The forward transmitted residual blue light and emitted amberlight was collected by a small integrating sphere above the sample. Thecolor and conversion efficacy CE (forward lumens divided by the incidentblue powder, lm/W_b) were measured.

FIG. 4 shows the color distribution of amber ceramic platelets with 120,90 and 70 μm. For a standard thickness of 120 μm, the platelets have anaverage CE of 91.4 Im/W. As the thickness decreases down to 90 and 70μm, the CE increases to 96.4 and 98 Im/W respectively.

A wavelength converter with the filter layer according to the presentinvention was prepared. The phosphor material was a layer of amberceramics. The filter was composed of 17 sub-layers of alternativematerials with low and high refractive index, Al₂O₃ and TiO₂. (Table 1:filter design).

TABLE 1 Sub-layer no. Filter layer Thickness [nm] 1 Al₂O₃ 37 2 TiO₂ 43 3Al₂O₃ 63 4 TiO₂ 50 5 Al₂O₃ 62 6 TiO₂ 51 7 Al₂O₃ 66 8 TiO₂ 50 9 Al₂O₃ 6810 TiO₂ 49 11 Al₂O₃ 65 12 TiO₂ 51 13 Al₂O₃ 68 14 TiO₂ 48 15 Al₂O₃ 54 16TiO₂ 53 17 Al₂O₃ 127 Air

A reflectance curve simulated from the filter design in table 1 is shownin FIG. 5. It shows a high reflectance between 410 and 500 nm. Thereflectance drops quickly above 500 nm with a cut-off wavelength atabout 530 nm. Such coating can be made on amber ceramic platelets byelectron beam evaporation, sputtering or other coating method. An amberplatelet before and after the coating was measured by OSRAM Tester. Themeasurement results are compared to a simulated reflectance curve of thecoating in FIG. 5. After the coating, the residual blue light near 450nm is greatly reduced while amber emission near 600 nm is slightlyincreased.

FIG. 6 compares the color distribution of amber ceramic platelets with120, 90 and 70 μm before and after the coating. The Cx of all coatedplatelets increases. The thinner the coated platelets, the more their Cxincreases. The CE of coated platelets is about 1% higher than beforecoating the platelets.

Coated platelets were assembled in a LED package together with uncoatedproduced amber platelets as reference. The emission color of coatedamber platelets in the package (FIG. 7) is very similar to colormeasured by Osram tester in FIG. 6. The package lumens @700 mA forcoated amber platelets are 5-18% higher than reference productionuncoated platelets (FIG. 8).

In some embodiments, the amber ceramic surface finishing is a furtheraspect of the coating process. FIG. 9 shows a SEM image of a crosssection of a coated amber ceramic. The ceramic surface is as ground, notvery flat. The imperfection of the surface can negatively affect thecoating in two ways. First, it can reduce the coating quality. Secondly,it can introduce a scattering which will affect the incident angle oflight to the coating. The reflectance of the coating can depend on theincident angle of light. As the incident angle increases, thereflectance curve in FIG. 5 will shift to left and the cutoff wavelengthwill decrease. So the scattering caused by the imperfection of thesurface can change the reflectance/transmittance. It is preferred tohave a polished surface of amber ceramic for the coating purpose.

The amber platelets with 70, 90 and 120 μm thickness were polished downto 1 μm with a diamond slurry. Then the polished platelets and as groundplatelets were coated with the layers according to table 2.

TABLE 2 Sub-layer no. Filter layer Thickness [nm] 1 Al₂O₃ 32.7 2 TiO₂53.1 3 Al₂O₃ 55 4 TiO₂ 51.3 5 Al₂O₃ 67 6 TiO₂ 53.9 7 Al₂O₃ 60.9 8 TiO₂56 9 Al₂O₃ 68.1 10 TiO₂ 46.4 11 Al₂O₃ 71.1 12 TiO₂ 59.6 13 Al₂O₃ 48.9 14TiO₂ 52.4 15 Al₂O₃ 63 16 TiO₂ 57.1 17 Al₂O₃ 128.8 Air

The coated polished and un-polished platelets were measured by OSRAMtester (FIG. 10). All the polished platelets color is more saturatedthan unpolished platelets, which is indicated by an increase of Cx,which means there is less residual blue in coated polished platelets.Their CE is also 1-2% higher.

Another embodiment is a stack of a thin assembly of a phosphor layer anda filter layer with another transparent layer, preferably glass, AI₂O₃,or Silicone (FIG. 11). Between the transparent layer 5 and the phosphorlayer 2 is the filter layer 3. The filter layer can be applied to thetransparent layer 5 in high volume processes first and then glued to thephosphor layer 2 (e.g., (Sr,Ba)₂Si₅N₈ amber ceramics) by transparentepoxy, silicone, or polysiloxane, water glass or a low melting glass.

The advantage of this embodiment is that a transparent layer, such asglass, can have a much smoother surface than ceramics. Therefore, thefilter layer on such a phosphor layer can have a better quality. Sincethe disclosure uses thinned amber ceramic with less scattering, onepossible issue is the ceramic's height is lower than maximum point ofbonding wire (FIG. 12). The highest portion of wire cannot be covered bycasting protection material, such as silicone. So another advantage ofthis embodiment is that the total thickness of the hybrid can beadjusted such that it is higher than the bonding wire, so the wholebonding wires can be protected by sidewall protection casting in thepackage (FIG. 12).

FIG. 13 shows an exemplary method for preparing a wavelength converter.In a first step a filter layer 3 is deposited on a glass substrate or asapphire wafer 8. The phosphor layer 2 is attached to the filter layerwith a glue 9. This leads to a so-called multilayer stack. In a furtherstep, the multilayer stack is diced into smaller pieces in a subsequentstep.

FIG. 14 shows an exemplary method for preparing a light emitting deviceassembly 6. In a first step, the wavelength converter 1 is attached tothe LED die 7, which is attached to a leadframe 10. A bonding wire 11 isattached to the LED die 7. In a subsequent step, the assembly is moldedwith a mold tool 12 in a transfer molding step and a protection siliconecasting 13 is present. After the molding step a light emitting deviceassembly 6 is obtained.

FIG. 15 shows an exemplary thick ceramic, while FIG. 16 shows aninventive thin ceramic. The conversion of blue light into light oflonger wavelength in the converter ceramics creates heat by Stokes shiftlosses. This leads to temperature rise in the ceramics element. Thethermal resistance of the converter element and therefore the maximumtemperature increases with thickness. Conversion efficiency is reducedwith temperature, degradation increased. Therefore reducing thethickness of the ceramics improves the performance of the conversion LEDparticularly in high power applications with current densities >1 A/mm².Therefore a thin ceramics layer d<70 μm with coating is better than athick layer. In summary R_(th_thick)>R_(th_thin) leads toT_(thick)>T_(thin).

FIG. 17 shows the reflective properties of a coating. A problem is thatthe reflection band of dielectric coating shifts in wavelength withangle of incidence. 100% reflection of broad blue LED (FWHM-25 nm)cannot be secured for all angles of incidence. Therefore, the color isless saturated, e.g., for high angles of incidence. A possible solutionis to add a wavelength selective absorptive layer on top of the filterlayer, which leads to an absorption of blue light and therefore to thetransmission of light of a longer wavelength. The majority of unabsorbedblue light (e.g., >90%) is reflected by the filter layer, the residualpart absorbed by the blue filter. The filter layer band and blue LEDshould be tuned such that vertical incidence is reflected best andhigher angles are partially transmitted, since the optical path lengthin the absorptive layer is longer for higher angles of incidence.

FIG. 18 shows the color of an LED with ceramics layer as seen fromdifferent observation angles. 0° is for observation along the opticalaxis, larger angles for oblique observation. While for a thick ceramicslayer without mirror (2) the color shift between direct and obliqueobservation is low, it can vary significantly for the case of a thinceramics layer with a dielectric filter (1). This case may requireblocking of light from oblique angles, additional mixing optics, or asdescribed an additional absorbing layer on top of the reflective mirror.

FIG. 19 shows different embodiments of wavelength converters 1. Anabsorption layer 4 could be directly deposited on top of the filterlayer 3, or on the lower or upper side of the glass substrate orsapphire wafer 8. Another option is to coat a blue filter glass with thefilter layer 3. Materials for the absorption layers can comprise: iondoped glasses (Schott filter glasses), converter materials with highactivator concentration and high quenching, or particularly for thincoatings semiconductor materials with band gap larger than the emission(e.g., GaP or similar for yellow).

With the methods and means described herein, the advantage of higherefficacy of thinner ceramics and meanwhile to make sure its color fallsinside of color requirements.

In general thick layers of wavelength converters are often inefficientbecause of scattering, however they often have a high color saturation.It is often that the thinner the wavelength converters are, the lessscattering occurs in combination with a higher efficiency, but a lowcolor saturation. The wavelength converters of the present inventionshow low scattering, low back reflection, a high efficiency and a goodsaturation.

While there have been shown and described what are at present consideredto be preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention as definedby the appended claims. The disclosure rather comprises any new featureas well as any combination of features, which in particular includes anycombination of features in the appended claims, even if the feature orcombination is not per se explicitly indicated in the claims or theexamples.

What is claimed is:
 1. A wavelength converter comprising: a phosphorlayer; and a filter layer, wherein the filter layer is directly attachedto the phosphor layer, and wherein the wavelength converter has anoverall thickness of between 20 μM to 80 μm.
 2. The wavelength converterof claim 1, wherein a phosphor of the phosphor layer is selected fromthe group consisting of (Ba,Sr)₂Si₅N₈:Eu²⁺, Ca-α-SiAlON:Eu²⁺,YAG:Ce+CaAlSiN₃:Eu, (Ca,Sr)AlSiN₃:Eu²⁺, (Sr,Ca)Al₂Si₂N₆:Eu²⁺,(Ba,Sr,Ca)₂Si₅N₈:Eu²⁺ and Sr(LiAl₃N₄):Eu²⁺.
 3. The wavelength converterof claim 1, wherein the filter layer comprises at least two differentmetal oxides.
 4. The wavelength converter of claim 1, wherein the filterlayer comprises at least two different metal oxides selected from thegroup consisting of SiO₂, Al₂O₃, TiO₂, Nb₂O₅, Ta₂O₅, HfO₂, and Y₂O₃. 5.The wavelength converter of claim 1, further comprising an absorptionlayer.
 6. The wavelength converter of claim 5, wherein the absorptionlayer is selected from ion doped color filter glasses comprising MoS₂dyed glasses, Ce-doped Gallium-Gadolinium-YAG, or is selected fromsemiconductor materials.
 7. The wavelength converter of claim 6, whereinthe semiconductor materials comprise GaP, AlP, AlAs, CdSe or CdS in formof thin layers or in form of nanoparticles.
 8. A light emitting deviceassembly comprising: a LED die; and a wavelength converter comprising: aphosphor layer; and a filter layer, wherein the filter layer is directlyattached to the phosphor layer, and wherein the wavelength converter hasan overall thickness of between 20 μm to 80 μm.
 9. A method forpreparing a wavelength converter, the method comprising: providing aglass substrate or a sapphire wafer, coating the glass substrate or thesapphire wafer with a filter layer comprising different metal oxides toprepare a coated glass substrate or coated sapphire wafer, providing aphosphor material, and attaching the coated glass substrate or coatedsapphire wafer to the phosphor material, thereby providing a wavelengthconverter comprising a phosphor layer, wherein the filter layer isdirectly attached to the phosphor layer, and wherein the wavelengthconverter has an overall thickness of between 20 μm to 80 μm.
 10. Themethod according to claim 9, wherein the metal oxides are selected fromthe group consisting of SiO₂, Al₂O₃, TiO₂, Nb₂O₅, Ta₂O₅, HfO₂, and Y₂O₃.11. The method according to claim 9, wherein the phosphor material isselected from the group consisting of (Ba,Sr)₂Si₅N₈:Eu²⁺,Ca-α-SiAlON:Eu²⁺, YAG:Ce+CaAlSiN₃:Eu, (Ca,Sr)AlSiN₃:Eu²⁺,(Sr,Ca)Al₂Si₂N₆:Eu²⁺, (Ba,Sr,Ca)₂Si₅N₈:Eu²⁺ and Sr(LiAl₃N₄):Eu².
 12. Themethod according to claim 9, wherein attaching the coated glasssubstrate or the coated sapphire wafer to the phosphor materialcomprising laminating the coated glass substrate or the coated sapphirewafer to the phosphor material with a glue.
 13. The method according toclaim 9, further attaching an absorption layer to the wavelengthconverter.
 14. The method according to claim 9, further comprisingdicing the wavelength converter.
 15. A wavelength converter preparedaccording to the method of claim
 9. 16. A method for preparing a lightemitting device assembly, comprising: providing a LED die; and attachinga wavelength converter to the LED die, the wavelength convertercomprising a phosphor layer and a filter layer, wherein the filter layeris directly attached to the phosphor layer, and wherein the wavelengthconverter has an overall thickness of between 20 μm to 80 μm.
 17. Themethod according to claim 16, wherein the LED die is selected fromGaN/InGaN based semiconductor materials.
 18. A light emitting deviceassembly prepared according to the method of claim 16.